This section Jeremiah 17:1-4 is inseparably connected with the preceding. Judah’s sin had been described Jeremiah 16:19 as one of which the very Gentiles will become ashamed. and for which she will shortly be punished by, an intervention of God’s hand more marked than anything in her previous history. Jeremiah now dwells upon the indelible nature of her sin.
A pen of iron - i. e., an iron chisel for cutting inscriptions upon tables of stone.
The point of a diamond - The ancients were well acquainted with the cutting powers of the diamond.
Altars - Not Yahweh’s one altar, but the many altars which the Jews had set up to Baalim Jeremiah 11:13. Though Josiah had purged the land of these, yet in the eleven years of Jehoiakim’s reign they had multiplied again, and were the external proofs of Judah’s idolatry, as the table of her heart was the internal witness.